Recently, mobile devices are steadily being reduced in weight and miniaturized, and, at the same time, are gradually being highly functionalized while providing a variety of functions. Accordingly, secondary batteries have gradually increasing capacity and voltage, and secondary batteries having long cycle lifespan, excellent stability at high potential, and excellent storage characteristics at high temperature are required.
Among lithium transition metal oxides broadly used as a cathode active material of lithium secondary batteries, LiCoO2 is widely used due to excellent overall physical properties such as excellent cycle properties, and the like. However, LiCoO2 is low in safety and expensive due to resource limitations of cobalt as a raw material. In addition, LiCoO2 has a variety of problems such as a low charge and discharge current capacity of approximately 150 mAh/g, an unstable crystal structure under a voltage of 4.3 V or more, ignition risk due to reaction with an electrolyte and the like.
Meanwhile, as a cathode active material for lithium secondary batteries, a lithium transition metal oxide, in which some nickel is substituted with a different transition metal such as manganese, cobalt, and the like, is proposed. However, such a nickel-based lithium transition metal oxide substituted with a metal has advantages such as relatively excellent cycle and capacity characteristics, but does not sufficiently solve problems such as dramatic reduction of cycle characteristics after extended use, swelling event due to gas generation, low chemical stability and the like.
Regarding the above problems, impurities formed by a manufacturing raw material of an active material remaining in a nickel-based lithium transition metal oxide reduce battery capacity, or the impurities are decomposed and thereby generate gas, and, accordingly, in batteries, swelling occurs.
As one solution to the above problems, a technology to use a mixture of two or more different lithium transition metal oxides as a cathode material has been proposed. When the mixture of two or more different lithium transition metal oxides is used, drawbacks of separately using each lithium transition metal oxide may be overcome. However, the prior cathode material mixtures are prepared by simply mix two ingredients and thereby synergy effect beyond simple mixing may not be anticipated.
Therefore, there is an urgent need to develop a technology which is suitable for high capacity secondary batteries and may solve problems with stability at high temperature.